The price range on the public domain kbooks is just publishers trying to make a buck. Maybe value added by formatting and book cover; maybe.
One warning here. If you own an iphone/touch, the ebook from another source cannot be transferred to the iphone/touch with the new Amazon Kindle app. If the kbook is from Amazon, you can read on both the Kindle and iphone/touch and stay in sync. Thus you might want to check for a free version at Amazon first.